Sheet materials having three-dimensional surfaces are well known in the art and have been utilized in many different applications. Such materials may be made in planar sheet form and remain in that form throughout handling or they may be made in continuous web form and be wound on rolls for handling. When in planar form, nesting of sheets may be useful to reduce stack height, for example. However, when wound onto rolls, nesting frequently causes significant problems. For example, nesting of layers having three dimensional surfaces on a roll may create difficulty unwinding the roll due to frictional engagement. Also, if nesting occurs more deeply at one end of the roll than the other end, roll telescoping may occur.
Nesting of any three-dimensional material can occur if protrusions in overlying webs or portions of the same web interlock with one another due to their size, shape, location, and/or geometrical arrangement. Two types of nesting or interlocking can occur: face-to-face nesting and face-to-back nesting. Face-to-face nesting can occur when two analogous faces of the same web or faces of plural webs are brought into contact and protrusions of each web or web portion enter valleys or spaces between adjacent protrusions of the other web or web portion. Face-to-back nesting can occur when opposing sides of the same web or dissimilar sides of plural webs are brought into contact and protrusions of one web or web portion enter the hollow "negative" side of protrusions of the other web or web portion. Such face-to-back nesting is of particular concern with three-dimensional sheet materials having hollow three-dimensional protrusions such as those of the present invention. If a web only exhibits three-dimensional surface features on one surface, i.e., the back surface is generally planar, then face-to-face nesting emerges as the primary consideration as face-to-back nesting cannot occur. While face-to-face nesting is more easily cured by selection of a protrusion spacing which is less than the width of the smallest protrusions, such a selection seriously limits the versatility of the resulting structure since such small protrusion spacing may be undesirable for some applications.
Nesting of adjacent layers or windings of a continuous web when rolled can create difficulty in unrolling the end of the web due to the frictional engagement which occurs with face-to-back nesting. Nesting of stacked individual layers or plies of three-dimensional web materials can result in removal of multiple sheets from a container when only a single sheet is desired, etc. While the ability of the web to nest with itself or with other webs may create a certain degree of difficulty in and of itself, where the three-dimensional web structure is utilized as a carrier for an active substance such as, for example, an adhesive, nesting can cause additional difficulties including premature adhesion and/or contamination of the active substance.
In the art of tapes, labels, and other articles using pressure sensitive adhesive to adhere an adhesive coated surface to a target surface, there has been recognized the problem of premature adhesion. That is, before the adhesive coated surface can be properly positioned over a target surface, inadvertent contact of the adhesive with the target surface causes premature adhesion at one or more locations, thereby inhibiting proper positioning. Others have attempted to overcome this problem by providing an initial contact surface spaced outwardly from the adhesive surface. The initial contact surface acts as a stand-off from the adhesive surface. This is achieved, for example, by a process that forms conical protrusions in a film and partially filling the valleys between protrusions with adhesive at a level below the tips of the protrusions. One such three-dimensional sheet material, as well as methods and apparatus for manufacturing same, is described in greater detail in commonly-assigned, co-pending U.S. patent application Ser. No. 08/584,638, now U.S. Pat. No. 5,662,758 entitled "Composite Material Releasably Sealable to a Target Surface When Pressed Thereagainst and Method of Making", filed Jan. 10, 1996 in the names of Peter W. Hamilton and Kenneth S. McGuire, the disclosure of which is hereby incorporated herein by reference.
While such three-dimensional sheet materials are useful for their intended use, there remains to be solved the issue of nesting of adjacent layers or windings of such materials during manufacture, storage, and dispensing prior to use.
Attempts have been made in the prior art to address the tendency of three-dimensional structures to nest with one another, particularly in the instance of superimposed sheet materials with hollow three-dimensional protrusions, by randomly or at least non-uniformly locating similarly-sized and shaped protrusions on the web. While this approach may be effective for certain web constructions, its effectiveness decreases as the total land area (free space without protrusions) decreases. This is due to the fact that, given an essentially constant protrusion size and shape, increasing the number density of protrusions decreases the degree of possible spacing and location variations to the point where, in a limiting case, the protrusions become so closely packed that they approach a closely-packed uniformly-spaced array. Moreover, due to the non-uniform placement of the protrusions the resulting physical properties of the web are also non-uniform and may be difficult to tailor if the placement is not predictable.
Accordingly, it would be desirable to provide a three-dimensional sheet material which resists nesting of one layer with another superimposed layer whether in planar sheet form or in rolled form.
It would further be desirable to provide a method for forming such three-dimensional sheet materials which may be suitably tailored for a particular material need and which may be readily and economically practiced.
It would also be desirable to provide a suitable forming apparatus for forming such three-dimensional sheet materials which may be tailored for a particular material need and which may be readily and economically utilized in commercial practice.